The present invention relates to the controlled guidance of a d.c. motor into a target, or destination, position through the use of a digital control and a position sensor associated with the motor shaft for emitting a displacement step signal in response to each movement of the shaft through an incremental angle constituting a given fraction of one cycle of shaft rotation, by comparing a signal derived from such step signal with a control signal and utilizing the result of the comparison to control the motor drive current.
Various methods and circuit arrangements for bringing a motor into the vicinity of a target position by braking are already known. However, in order to bring the motor to the target position with high accuracy, special measures must be taken during the last phases of movement. Such measures are, for example, to let the motor travel to the target position, after braking, in a creeping movement or to adjust the braking delay so that it approaches a given curve.
In the first case, the speed selected for the creeping movement must be very low so that when the step instruction is finally given at the target position there will be no overshoot. Since, moreover, the creeping movement must start early enough so as to be independent of differences in load and friction, it takes a relatively long time for the motor to reach the target position. In the second case, a large amount of circuitry is required, particularly because digital signals must be converted to analog signals, to make it possible to approach the inherent analog delay characteristics.
A solution of the latter type is disclosed in German Offenlegungsschrift [Laid-open Application] No. 2,264,323 in which a clock pulse is derived from uniformly spaced path markers and is then used to obtain the analog value representing the remaining portion of the path. From this it is determined whether the ratio of actual speed to remaining path during braking has a given value with linear reduction in speed. Deviations are corrected by braking, idling or acceleration.
This method employs relatively complicated means to obtain analog values from digital circuit elements and signals and is dependent, inter alia, on the accuracy of constant current sources. Since, moreover, it is able to provide only a linear delay characteristic, the speed of the motor does not gradually die down as the target position is reached and the motor experiences overshooting or at least a sudden stop.